The present invention relates to reinforcing foams for strengthening structural elements that contain a cavity.
Manufacturers are continually looking for ways to reduce the weight of automobiles. A common approach is to replace heavy metal parts with lighter plastic ones. This works well when the part does not bear heavy loads and is not subjected to great mechanical stresses during production and use. However, it is difficult to replace a metal part with a plastic one when the part is called upon to withstand high loads or mechanical stresses. The mechanical properties of plastics rarely match those of metals.
Therefore, a compromise approach has been developed which permits lighter-weight metal parts to be substituted for heavier ones. This approach makes use of a structural member, typically a metal, which is reinforced with a polymer foam. The polymer foam is inserted into a cavity in the structural member and an adhesive secures the foam to the structural member. This approach allows thinner-gauge or smaller metal parts to be used, with the resulting loss of mechanical properties being at least partially compensated for by the structural foam insert. In some cases, this approach allows the complete replacement of metal with lighter-weight polymers.
It is possible to form the reinforcing polymer foam in place within the cavity of the structural member. This technique is often used for applying spot reinforcement on vehicle assemblies or sub-assemblies having accessible cavities. A curable foam formulation is poured, sprayed or injected into the cavity and cures in place to form the reinforcement.
It is often convenient for manufacturing reasons to form the structural member and polymer foam separately, and then affix the foam to the structural member in a separate step. This can often be done by forming a polymer foam insert that is slightly smaller than the cavity it will fill. All or part of the foam surface is covered with a layer of a thermally expandable adhesive. The resulting structural foam insert (“SFI”) is then placed within the cavity and heated so that the expandable adhesive expands, filling the remaining space in the cavity and adhering the polymer foam to the structural member. This process is described, for example, in U.S. Pat. No. 5,194,199 to Thum, U.S. Pat. No. 5,755,486 to Wycech, U.S. Pat. No. 5,806,919 to Davies and U.S. Pat. No. 6,068,424 to Wycech, among others. Automobile manufacturers usually incorporate the thermal expansion step into a primer curing step.
A polyurethane foam is commonly used in these reinforcing applications, either as a foam-in-place reinforcement or as the core of these SFIs. The polyurethane has the advantage of being a thermoset, which makes it less likely to melt or flow when exposed to elevated temperatures and also tends to increase its dimensional stability. Polyurethane foams also can be polymerized, expanded and shaped to fit within the cavity in a single processing step. Wasteful fabrication steps that are needed with thermoplastic foams can be avoided, which reduces the cost of the insert.
The polymer foam core often is subjected to substantial stresses over the life of the part. The stresses can arise from a number of factors, including thermal expansion and contraction, vibrations and impact events, and exposure to fluids. Sometimes microcracks or even larger defects develop as a result of these conditions. In the case of SFIs, very substantial stresses are incurred during the step of expanding the adhesive. The thermal expansion and contraction of the structural member and of the foam itself due to heating and cooling can also induce stresses. These stresses sometimes cause polyurethane foam reinforcements to crack or break apart, weakening the entire assembly.
In addition, the polyurethane foams that are used in these applications often exhibit a significant drop-off in mechanical properties at higher use temperatures. This weakens the structure at higher temperatures such as might be encountered during summer months or in other hot environments. Some parts that are located near hot engine parts can experience these temperatures even during the winter in cold climates.
It would be desirable to provide a polymer foam reinforcement that is more resistant to cracking or other mechanical breakdown during the step of expanding the adhesive layer. It would be further desirable a polymer foam reinforcement that retains more of its mechanical strength at temperatures in the range of 45-100° C.